Black Cherry—aphis or fly (Myzus cerasi).—This aphid has now a world-wide distribution. In New Zealand it has been found on cherry and plum, though in other countries its hosts include peaches, red and black currants, and cruciferous plants, such as common mustard, shepherd’s purse, etc. This species exudes copious honey-dew, upon which sooty mould develops, thus rendering fruit unfit for use. The principal injury, however, is due to the destruction of shoots and leaves, the latter frequently curling up when the insect clusters in dense colonies upon the infested plant. The complete life-cycle has not been followed under New Zealand conditions, but the shiny black eggs occur on the bark and buds of cherry trees during the winter. In spring the eggs hatch, and the insects, rapidly reproducing, attack the young shoots and leaves. Observers in other countries have noted that there is a summer migration of winged females to cruciferous plants, where colonies are established, and whence there is a return migration during the autumn to the original host. The wingless females are black, with part of the legs yellow, while the young individuals are pale in colour; the winged females have a green abdomen, from which arise the black “honey-tubes.” Since all the over-wintering eggs have hatched by the time the buds open, the insect can be then controlled by applications of nicotine-sulphate.
FIG. 9.
(1) Peach leaves attacked by Black Peach aphis. (2) Colony of Cabbage aphis on leaf. (3) Stem of insignis pine attacked by Chermes. (4) Grape Phylloxera and galls on vine roots. (5) Grape Phylloxera galls on vine leaf. (6) Woolly aphis on apple twig. (7) Galls of Poplar aphis. (Figs. 1, 2 and 6 by W. C. Davies; Fig. 4. after U.S. Dept. Agric.; Fig. 5. after N.Z. Dept. Agric.)
Cabbage Aphis (Brevicoryne brassicæ).—The cabbage aphis, or cabbage green fly, is widely distributed throughout the world, and has become a serious pest in New Zealand, causing considerable damage to cruciferous crops; it infests rape, turnip, cabbage, Brussels sprouts, cauliflower, as well as related weeds, such as wild mustard, shepherd’s purse and watercress. Most damage is done during dry seasons, when the plants succumb more readily to attack; if the insects are numerous, they cause the leaves to curl, and give a greyish appearance to infested plants, which may become flaccid and sticky from the copious honey-dew of the insect. The wingless forms are bluish in colour and coated with a greyish powder, but the winged females have the head and thorax black and the abdomen greenish ([Fig. 9], 2). In New Zealand all stages may be found throughout the year on winter crucifers or on weeds, though reproduction is retarded during the winter; in the spring the winged females fly to young crops. In very cold climates eggs are laid in the autumn, and these survive the winter. The cabbage aphis is attacked by a number of parasites, and usually the brownish empty shells of a large number that have been destroyed by a small parasite are to be found at any time; other important enemies are the hover-flies, the eleven-spotted ladybird beetle, and the Tasmanian aphis-lion. The insect can be controlled by spraying with nicotine-sulphate to which soap has been added.
Pine Tree Chermes (Chermes pini).—This is a widely-distributed species, occurring upon both Austrian and insignis pine in New Zealand. The insect lives in colonies upon the cones, twigs and branches, as well as around the bases of the needles; each aphis exudes a woolly covering, which forms conspicuous white masses when the trees are heavily infested ([Fig. 9], 3). Young trees seem to be the more subject to infestation, from which they may recover as they grow, but some damage is caused by the insect by a weakening of the trees, especially where grown in unsuitable localities. It is frequently noticed that individual trees in a plantation are heavily infested, while adjacent trees of the same species are not. The wingless form of the insect, covered by its mat of white threads, is brownish in colour and ornamented with numerous dark spots; there are no “honey-tubes” on the abdomen. The life-cycle of this insect becomes complicated, when it develops on two types of conifers; in the latter case the primary host is a species of spruce upon which the insect forms galls, and the secondary host may be larch, Douglas fir or pine, upon which gall formation is unusual. So far as is known, only the pine-infesting form of the aphis occurs in New Zealand.
Grape Phylloxera (Phylloxera vastatrix).—This destructive aphis, sometimes called the grape louse, is a native of North America, where it normally infests grape vines. It was accidentally introduced into the grape-growing districts of France, where it became very destructive. It later made its appearance in New Zealand. The insect infests both the leaves and roots of grape vines, the root-feeding stages being the most destructive, in consequence of which vines are now grown on resistant root stocks. The leaf-infesting stages of the insect cause pocket-like galls to form, which open on the upper surface of the leaf by a narrow aperture concealed under a tuft of delicate hairs ([Fig. 9], 5). In each gall the aphid matures and deposits several hundreds of eggs, from which wingless females hatch; these wander to other leaves, and each insect forms a new gall for itself. Several generations develop thus, but later many of the offspring migrate underground and join the root-infesting colonies. The irritation set up by the latter causes yellow flabby nodules to develop on the roots ([Fig. 9], 4). These nodules, or galls, later decay. The root-feeding aphides are wingless, and reproduce by means of eggs for several generations. Although they may go on developing thus for many years, it usually happens that, toward autumn, some of the insects transform to winged females, which fly to other vines or are carried thence by the wind. There each female feeds on the lower leaf surface, and deposits two kinds of eggs, some larger and some smaller; from the larger develop wingless females, and from the smaller wingless males, which are unable to feed. After fertilisation, each of these females deposits a single egg upon the older bark of the vine; such eggs do not hatch until the spring, when they give rise to the wingless females that start the galls on the leaves. Control depends on the use of phylloxera-resistant stocks, since it is from the root colonies of the aphis that the foliage is re-infested in the spring. An important feature is to prevent the scion from sending down roots where the union of the scion and root stock is close to the soil; if such scion roots form, they should be cut away and the soil removed from the union.
Rose Aphis (Macrosiphum rosæ).—The rose aphis is perhaps one of the best-known insects of the garden, mainly owing to its prevalence upon the young growth of all kinds of roses; it sometimes occurs on apple, tomato and rhododendrons. In a colony some of the insects are pink, and others bright green, though in the winged forms the head, antennæ, thorax, a row of spots on each side of the abdomen, and the “honey-tubes” are black; in both winged and wingless forms the eyes are red. In the case of severe infestations, plant growth is retarded and the leaves and flowers become distorted. Control can be effected by applications of nicotine-sulphate, kerosene, or soap solution.
Apple Woolly Aphis (Eriosoma lanigerum).—Although frequently called “American Blight,” the apple woolly aphis is probably a native of Europe. It occurs throughout New Zealand, and was a very serious pest until controlled by the Aphelinus parasite. The presence of this insect is made apparent by the characteristic white woolly patches ([Fig. 9], 6) which appear upon the apple trees, due to the woolly material secreted by the aphid. Another feature is that the part of the tree attacked, even after the insects have disappeared, is disfigured by gnarled swellings, due to abnormal thickening of the inner bark. This species also infests apple tree roots, which become similarly malformed. However, root infestation has been overcome by using root stocks, such as Northern Spy, that are immune. The individuals comprising a colony of woolly aphis are variously coloured, yellow, green and red predominating; a considerable amount of honey-dew is secreted. This species has been found to migrate to the foliage of the elm and mountain ash, but in New Zealand the elm-infesting form has not been found to occur. The insect becomes active in spring, and rapidly increases until the autumn. Under favourable climatic conditions, winged females develop and produce males and females, the latter laying eggs. The woolly aphis is preyed upon by the nine-spotted ladybird, but, as this beetle is itself the victim of another insect, its utility is greatly minimised. The most important check to the aphis is the Aphelinus parasite (Aphelinus mali), the influence of which has been spectacular under New Zealand conditions.